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Decisions

Decisions. Chapter 6. The if Statement. Action based on a conditions If the condition is true, the body of the statement is executed if (amount <= balance) balance = balance – amount;. Flow Chart for If Statement. Amount <= Balance?. Look familiar?. balance=

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Decisions

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  1. Decisions Chapter 6

  2. The if Statement • Action based on a conditions • If the condition is true, the body of the statement is executed if (amount <= balance) balance = balance – amount;

  3. Flow Chart for If Statement Amount <= Balance? Look familiar? balance= balance - amount

  4. The if Statement (Java Form) If (amount < = balance) balance = balance – amount; else balance = balance – OVERDRAFT_PENALTY; Java Statements . Visual Logic . Amount <= Balance? balance= balance – OVERDRAFT_PENALTY

  5. If Else Flowchart

  6. Java Code for If Else • If (amount < = balance) • balance = balance – amount; • else • balance = balance – OVERDRAFT_PENALTY;

  7. The if Statement • What if you need to execute multiple statements based on decision? if (amount <= balance) { double balance = balance – amount; }

  8. What Is Wrong if (amount <= balance) newBalance = balance – amount; balance = newBalance; if (amount <= balance) { newBalance = balance – amount; balance = newBalance; }

  9. Relational Operators • Test the relationship between two values. • Most of the operators are similar to Visual Logic • There are some differences. A big one is equals. • In Visual Logic equals is =. In Java ==. • Not equal in Java is !=.

  10. Relational Operators Note: Equal and Not equal are different in Java than Visual Logic!

  11. Comparing Floating-Point Numbers • Must be careful in comparing • Rounding problems

  12. Java Example of Floating Point Comparison

  13. Comparing floating – Point Numbers • Need to test for close enough final double EPSILON = 1E-14; if (Math.abs(x-y) <= EPSILON) // x is approximately equal to y

  14. Comparing Strings • Can’t use mathematical operators • Special methods have been developed if (string1.equals(string2))…. • Remember case matters • Special method if (string1.equalsIgnoreCase(string2))…. This is very different from Visual Logic

  15. Comparing Strings • May want to compare other than equal string1.compareTo(string2) < 0; Compares based on dictionary • What happen if we use == • We are testing the string on left with constant If (nickname == “Rob”) • Testing to see if the variable nickname value is Rob. Will be true only if it points to the string object Remember that a string is and object and what is stored is the address. That is the problem

  16. Comparing Strings • String nickname = “Rob”; • If(nickname == “Rob”); //Test is true • String name=“Robert”; • String nickname = name.substring(0,3); • If(nickname == “Rob”); //Test is false

  17. Comparing Objects • Use == you are testing to determine if the reference of the two objects are the same. • In other words, do they point to the same address Rectangle box1 = new Rectangle(5, 10, 20, 30); Rectangle box2 = box1; Rectangle box3 = new Rectangle(5, 10, 20, 30); box1 == box2; // true or false box1 == box 3; //true or false

  18. Testing for Null • Null is a special object reference • It says no value has been set. • The memory has been set aside or instantiated • Null is not the same as the empty string “” • “” assigns a blank • Null means no assignment has been made if (x = null)………

  19. Multiple Alternatives • Require more than 1 if/else decisions • Need to make series of related comparisons • Use if …. else if ……

  20. Example public String getDescription() { if (richter >=8.0) r = “Most structures fall”; else if (richter >= 7.0) r = “Many buildings destroyed”; ………. else r= “Negative numbers are not valid”; return r; }

  21. Switch Statement • Use instead of a sequence of if/else/else statements. int digit; ….. Switch (digit) { case 1: System.out.print(“one”); case 2: System.out.print(“two”); default: System.out.print(“error”); }

  22. Nested Branches • Based on the decision of one statement make another decision.

  23. Nested Branch in Visual Logic

  24. Nested Branches - Java If (status == SINGLE) { if (income <= SINGLE_BRACKET1) tax = Rate1 * income; else if (income <= SINGLE_BRACKET2) tax = Rate2 * income.

  25. Enumeration Types • Think of it as a switch statement for strings. • You assure that they are categorized correctly public enum FilingStatus {SINGLE, MARRIED} FilingStatus status = FilingStatus.SINGLE; • Use the == to compare enumerated values if (status==FilingStatus.SINGLE)…

  26. Using Boolean Expressions • True/False • Assigned to an expression such as <1000 double amount = 0; System.out.println(<1000); Output: true

  27. Predicate Methods • Method that returns a boolean value public class BankAccount { public boolean isOverdrawn() { return balance < 0; } }

  28. Character Class Predicate Methods • isDigit • isLetter • isUpperCase • isLowerCase • Scanner class • hasNext() • nextInt

  29. Boolean Operators • Test two conditions • and && • or || if (input.equals(“S”) || input.equals(“M”))….. If(input.equals(“S”) && input.equals(“M”)) …. • Can use ! – means not • Negates the answer Note repeat Same as Visual Logic

  30. Boolean Variables • Primitive type • Must declare as boolean private boolean married; if (married) ….. • Don’t use if (married == true)…

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